The invention is directed to an electrical surge protector having a protective enclosure to inhibit catastrophic failure of the surge protector.
Surge protection devices are used to protect components and power systems from prolonged overvoltage surges, such as those caused by lightning for example. During a prolonged overvoltage surge, a surge protection device provides temporary surge protection of a component by shunting the overvoltage surge to ground or neutral. A prolonged overvoltage surge may cause a surge protection device to overheat, thus presenting a fire hazard.
Very high voltage surges, such as those caused by nearby lightning strikes, can cause violent failure of a surge protection device, raising the temperature of the surge protection device and adjacent areas to levels well above combustion temperature for most materials. This high temperature, combined with integral or adjacent combustible materials, and the oxygen in the air adjacent the device can combine in an exothermic reaction causing burning of the device, and subsequent catastrophic damage to equipment, buildings, and personnel.
One method that has previously been used to overcome this danger is to place an enclosure around the components of the surge protection device, and then fill the cavity surrounding the components with a combustion-retardant substance, such as dry electrical-grade silica. The dry electrical-grade silica displaces the air (which contains oxygen) within the cavity, eliminating a critical component of the exothermic reaction, and also provides thermal capacitance to absorb the heat generated by the transient voltage spike, thus protecting adjacent equipment, even if the surge protection device is destroyed.
Recently, perhaps because of power utility deregulation, more power "swells," events where sustained power line voltages greater than the nominal power voltage plus normal regulation (e.g., 240 VAC +10%), are occurring. When these swells exceed the maximum continuous operating voltage of the surge protection device, the device conducts high currents for sustained periods of time, causing prolonged heating and subsequent failure of the enclosure, even when it is filled with dry electrical-grade silica. After the enclosure fails, air permeates the environment, providing oxygen for combustion, and serious fires may result.
Surge protection devices often contain indicator lights, such as light emitting diodes, which are lit when the surge protection device is intact, but extinguish when the device has failed. Since the indicator light is inside the housing of the surge protection device, a viewing port or hole, either exposing the indicator light, or covered by a clear membrane, is provided so that an observer may view the light. If a surge or swell occurs when the viewer is looking at the light, the hot gas generated by the event will expand rapidly and vent through the viewing port, potentially causing bodily harm to the viewer.